A so-called Gilbert cell or four-quadrant multiplier is a cross-coupled differential amplifier having a gain that can be linearly controlled by modulating emitter bias current or by adjusting the ratio of emitter degeneration resistance to the load resistance. The amplitude of the differential input RF signal can be linearly controlled by a differential AC voltage. Typical Gilbert cell configurations can be found, for example, in U.S. Pat. No. 3,689,752.
Gilbert cells are commonly used in a number of applications, including mixers, automatic gain control (AGC) amplifiers, amplitude and sideband modulators, amplitude modulation (AM) and sideband detectors, frequency doublers and dividers, squaring and square-root circuits. The typical implementation of a Gilbert cell mixer utilizes a simple differential pair as the input transconductance stage. This is cascode connected with the Gilbert switching quad, which is typically driven by a local oscillator signal. The resulting mixer can then be used to down-convert RF signals to some intermediate frequency (IF) in receiver applications, or to up-convert IF signals to the RF transmission band in transmitter applications.
Wideband receivers and transmitters often require multiple paths between the input and output because frequency-selective filters, often times in parallel, are used to cover different frequency bands and prevent signals at non-desirable frequencies from aliasing into the frequency band of interest at the output. Consequently, a M:1 switch is usually required in the system, either on the input of the mixer where M RF input signal paths are switched into a single mixer, or on the mixer output where the outputs from M mixers are switched to a signal IF output path. In addition, receivers and transmitters requiring multiple gain states require an additional variable gain amplifier or attenuator block to adjust the gain of the receiver, either at the input of the Gilbert cell mixer (in the RF band) or at the output of the Gilbert cell mixer (in the IF band).
However, cascading external switching and gain blocks (such as M:1 switches, amplifier, and attenuator blocks) in receivers and transmitters generally decreases system performance, including increased noise figure and decreased output intercept point (linearity). In addition, although some Gilbert cell multiplier circuits are themselves configured to receive multiple inputs, such conventional configurations necessitate multiplication of all the inputs at once, which is not suitable for all applications, such as those where only one of multiple inputs is desired for multiplication.